In the field of pressure vessels of nuclear reactors such as a BWR (Boiling Water Reactor) or a PWR (Pressurized Water Reactor), there are concerns about (1) residual tensile stresses remaining on the inner surface of welded joint portions where a nozzle and a pipe provided in the pressure vessel are joined together and the (2) occurrence of SCC (Stress Corrosion Cracking).
In the pressure vessels of nuclear reactors, stress improvement by reducing the tensile stress remaining on the inner surface of the welded joint portions and, preferably, generating compressive stress on the inner surface of the welded portions has been performed by IHSI (Induction Heating Stress Improvement) or shot peening (peening) with the objective of preventing the occurrence of SCC on the inner surface of welded joint portions.
With the IHSI, a high-frequency induction coil is disposed on the outer periphery of a welded joint portion, a high-frequency electric current is passed to the high-frequency induction coil to heat the welded joint portion and vicinity thereof, while causing a flow of cooling water within the welded joint portion (nozzle and pipe), a difference in temperature between the inner and outer surfaces that is necessary for stress improvement is created, and then the welded joint portion is returned to normal temperature, thereby reducing tensile stresses remaining in the inner surface of the welded joint portion and generating compressive stresses in the inner surface of the welded joint portion.
With the peening, the inner surface of a welded joint portion is peened by a large number of balls called shots, whereby the inner surface of the welded joint portion is plastically deformed, tensile stresses remaining on the inner surface of the welded joint portion are reduced, and compressive stresses are generated in the inner surface of the welded joint portion.
However, in particular, in a pressure vessel of a pressurized water reactor, there is no sufficient clearance between the outer periphery of the welded joint portion (nozzle and pipe) and the nuclear reactor containment vessel (shielding wall, etc.) and, therefore, it is difficult to install a high-frequency induction coil on the pressure vessel and perform the IHSI.
In particular, in the pressure vessel of a pressurized water reactor, the pressure vessel is located in the lowermost portion of the nuclear reactor containment vessel and the pressure vessel itself has a high doses of radiation. As a result, peening has to be performed in a state in which the pressure vessel is filled with water and, therefore, the peening apparatus is difficult to install on the pressure vessel and peening is difficult to perform.
Accordingly, clad welding (CRC: Corrosion Resistant Cladding), by which a clad layer with high corrosion resistance is formed on the inner surface of a welded joint portion with the objective of preventing the occurrence of SCC on the inner surface of the welded joint portion, can be considered instead of stress improvement such as IHSI and peening (see, for example, Japanese Patent Application Laid-open No. H10-197679).